Large amounts of finely divided material are routinely produced in industrial processes wherein particle size and/or moisture content within narrow limits are of importance. For example, chemical compounds typically react more readily in a finely divided state. The presence of water can be a factor in a chemical reaction. Accordingly, the characterization of a finely divided material can include limits as to particle size and water content within narrow limits. However, the determination of particle size and water content wherein the particles are of micron size by other than laboratory procedures is not easily obtained under production conditions. Typical of these materials produced in large quantities wherein particle size and moisture content are important are di- and tricarboxylic acids, polymers such as polyethylene and polypropylene. as well as others too numerous to mention.
Large amounts of aromatic di- and tricarboxylic acids are produced commercially by the oxidation of the alkyl aromatic compounds. Several different oxidation processes are known, with examples set forth in Volume 17, pages 732-777 of Kirk-Othmer's Encyclopedia of Chemical Technology, John Wiley and Sons, (1982). High molecular weight polyesters of these aromatic di- and tricarboxylic acids with various diols find extensive use in many applications. For example, high molecular weight polyesters of terephthalic acid have long been prepared of the Dacron.TM., Terylene.TM., and Kodel.TM. type fibers, and Mylar.TM. film, as described in U.S. Pat. No. 2,465,319 by transesterification of dimethylterephthalate with the appropriate gylcol followed by polycondensation. With the advent of improved processes for the manufacture of terephthalic acid of exceptionally high purity, the preparation of polyesters of terephthalic acid by direct esterification of terephthalic acid with glycol to a large extent has replaced dimethylterephthalate as a highly desirable alternate procedure.
This has manifest advantages of simplicity and economy as compared with the dimethylterephthalate route. Large quantities of terephthalic acid of suitable purity, free from contaminants are produced in a high-volume production procedure. Typically, many thousands of pounds of product are produced within a short period of hours. Analyses for contaminants by typical laboratory procedures can handicap high volume production.
Terephthalic acid is produced by the catalytic oxidation of paraxylene with molecular oxygen. Terephthalic acid is purified by a second process which consists of 1) catalytic hydrogenation to reduce certain oxidation by-products and 2) recrystallization to remove impurities. Water is used as a solvent. Because terephthalic acid is only sparingly soluble in water at ambient temperatures and pressure, but is soluble at high temperature and pressure, temperatures in the range of from about 400.degree. F. to about 700.degree. F. are used with high pressure. The difficulties of obtaining representative samples during the course of the process are obvious.
Crystal size is determined by crystallization temperature and concentration of terephthalic acid in the aqueous solution. Two or more crystallization stages are frequently used. Procedures for crystallization and extraction are taught in U.S. Pat. Nos. 3,082,250; 3,452,088; 3,497,552; 3,505,398; and 3,931,305, among others.
Esterification of terephthalic acid with a glycol is affected by particle size of the terephthalic acid. Particle size of the terephthalic acid also affects material handling characteristics of the material since the amount of so-called fines affects the bulk density of the material and the related properties such as the packing characteristics.
With the advent of optical analysis of impurity absorptions to determine impurity components present in both homogeneous and nonhomogeneous light scattering materials, see U.S. Pat. No. 4,755,048, much attention has been directed to determining levels of contaminants and particle size of materials present.
For example, the use of a laser beam in an apparatus for measuring particle size of particles in a fluid stream is taught in EP Patent No. 350768. Particle size and velocity measurement by measuring light scatter from a laser beam is taught in U.S. Pat. No. 4,854,705. Particle size in a liquid stream using a beam of light and recording radiation scattering is disclosed in SU Patent 1,448,246. The size of solid particles in a liquid phase is measured to control the size of the particles to allow continuous control of particle size in manufacture of a non-aqueous liquid cleaning product by the method of GB Patent 2,219,002.
Although the methods and instrumentation presently known in the art find widely differing applications, there is a need for a method to monitor in a production line the particle size of a material in a dry state wherein the particle size is from about 1 to 300 microns, and moisture content is a maximum of about 10 wt. %. In the case of terephthalic acid, variations in particle size within the range of from about 10 to 50 microns can be determined by very sophisticated reflectance spectrophotometric equipment such as Microtrac.TM., supplied by Leeds & Northup, St. Petersburg, Fla. However, this requires a laboratory procedure, poorly suited for an on-line analysis for a continuous production line for a critical product. Typically, thousands of pounds of product are prepared during the interval between analyses.
It is an object of this invention to provide a method for monitoring particle size of finely divided material, the particles of which range in size from about 1 micron to about 300 microns.
It is an object of this invention to provide a process for monitoring moisture content of finely divided material of up to about 300 microns in size and contains up to about 10 wt. percent moisture.
It is an object of this invention to provide a method for monitoring particle size of finely divided material in a series of samples, the particles of which, in part, range in size from about 1 micron to about 300 microns.
It is an object of this invention to provide an on-line method for continuous or periodic monitoring of particle size and/or moisture content of finely divided particles of up to about 300 microns in size and containing up to about 10 wt. % moisture.
It is an object of this invention to provide a continuous method for monitoring particle size of on-line production of terephthalic acid wherein said terephthalic acid comprises finely-divided material, the particles of which, in part, range in size from about 10 to about 50 microns.
It is an object of this invention to provide a continuous method for monitoring moisture content of particles of on-line production of terephthalic acid wherein said terephthalic acid comprises finely-divided material, the particles of which, in part, range in size from about 10 to about 50 microns.